The present invention relates to a fiber reinforced plastic (FRP) golf shaft, and more particularly, to a golf shaft that enables easy swinging of the golf shaft regardless of vibrations produced after impact, and to a golf club using such a golf shaft.
A typical golf shaft is made of FRP using carbon fibers for the reinforcing fibers. An FRP golf shaft may be manufactured through a known sheet winding process, a filament winding process, or a braiding process.
In the sheet winding process, synthetic resin is impregnated in ropings extending parallel to each other to form sheets of prepregs, which are cut into predetermined shapes. The prepregs are superimposed on a mandrel so that they are provided with the designed characteristics. The prepregs are hardened and then removed from the mandrel to form an FRP golf shaft. The properties, the orientation angles relative to the shaft axis, and the thickness of the prepregs are designed to realize the designed characteristics of the golf shaft manufactured through the sheet winding process. Such prepregs are arranged along the entire length of the golf shaft.
The cross-sectional thickness of the shaft, or the quantity of the prepregs, is constant so that the prepregs are isotropic in the radial direction. In some cases, the tip portion of the shaft, at which the head is connected, or the butt portion of the shaft that is closer to the grip may be partially reinforced. In such a shaft, the thickness of the shaft is substantially uniform except for the reinforced tip and butt portions. Further, the outer diameter of the golf shaft increases uniformly from the tip to the butt. Thus, the linear density along the axial direction of the shaft increases in a uniform manner from the tip portion to the butt portion.
In the filament winding process, fiber filaments are wound about a shaft forming mandrel to form a shaft. During the winding, the winding angle of the filaments relative to the shaft axis may be adjusted.
In the braiding process, resin is impregnated in fiber toes to form toe prepregs. The toe prepregs are then braided to form a shaft. Recent golf shafts (braiding shafts) are often manufactured through such process. Such a golf shaft has a high level in freedom of design with regard to flexural rigidity distribution and linear density distribution. In addition, such a golf shaft has satisfactory flexural strength and torsion strength.
There are a number of patent publications pertaining to golf shafts having linear density distributions that differ from normal shafts to enable the golf shafts to be swung more satisfactorily.
For example, Japanese Laid-Open Patent Publication No. 7-163689 describes a shaft provided with a mass formed by a balance weight. Japanese Patent No. 2622428 (corresponding to U.S. Pat. No. 5,716,291) describes a shaft having an outer diameter and an inner diameter that are changed in a sudden manner to partially expand the shaft. In both publications, the linear density is concentrated at portions excluding the tip portion and butt portion of the golf shaft, or at the central portion of the golf shaft.
However, the outer appearance, flexure feel, and strength of such a golf shaft are affected in an undesirable manner. More specifically, in the golf shaft of Japanese Laid-Open Patent Publication No. 7-163689 provided with the mass, stress concentrates at the boundary between the mass and the shaft when the golf shaft is swung. This decreases strength. Further, the golf shaft does not flex smoothly at the portion where the mass is added.
In the partially expanded golf shaft of Japanese Patent No. 2622428 (U.S. Pat. No. 5,716,291), the golf shaft does not flex smoothly depending on the amount of change in shape (cross-sectional secondary moment). Further, the outer appearance of the golf shaft is somewhat strange. Accordingly, although conventional golf shafts have theoretically ideal mass distributions, they are unsatisfactory from the viewpoints of outer appearance, flexure feel during swinging, durability, and manufacturing ease.
Japanese Laid-Open Patent Publication No. 2001-170232 describes a golf club that increases linear density by 20% at a portion located 0.322 to 0.605 meters from the grip end (in a section covering 30% of the club length, with the center of the section located at a position corresponding to 48% of the club length from the grip end). Further, the golf club has a club mass distribution that is optimal for the club length. As a result, the golf club is swung with more ease and the driving distance is increased with less work.
Japanese Laid-Open Patent Publication No. 2001-212273 describes a golf shaft in which the taper angle of the outer diameter is less than the taper angle of the inner diameter. This concentrates the linear density at portions other than the tip and butt of the golf shaft, or the central portion of the golf shaft. As a result, the golf shaft is provided with the optimal mass distribution without affecting the outer appearance of the golf shaft or the flexure feel of the swung golf shaft.
Furthermore, Japanese Laid-Open Patent Publication No. 2001-276288 describes a golf shaft in which the orientation angle of braiding yarns in braid layers relative to the shaft axis are changed depending on the axial position of the shaft. This concentrates the linear density at portions other than the tip and butt of the golf shaft, or the central portion of the golf shaft.
In the golf shaft of each of the above three patent publications, the change in linear distribution of the shaft that enables the golf shaft to be swung with more ease refers to concentration of the linear density at the central portion. Each golf shaft of the above three patent publications enables the golf shaft to be swung with more ease prior to ball impact. Further, the golf shaft is provided with the optimal mass distribution that eases swinging without affecting the flexure feel and strength of the shaft.
However, in such a golf shaft, when hitting the ball off-center or when hitting the ground instead of hitting the ball, the impact feel and the vibrations that are conveyed to the player's hands are somewhat uncomfortable.
Among vibration modes produced subsequent to impact, in the mode that becomes dominant, the antinodes of the vibrations are at the head and grip, and the node of the vibrations is at a portion extending from near the central portion of the shaft to a portion relatively near the tip. In a shaft having a structure in which the mass increases at the portion corresponding to the node of the vibrations, vibration tends to be amplified. This is one factor that causes discomfort.
It is an object of the present invention to provide a golf shaft that does not cause discomfort caused by the impact feel and the vibrations conveyed to the player's hands, and that is easily swung up until impact without affecting-in an undesirable manner the outer appearance of the golf shaft, the flexure feel of the golf shaft during swinging, and the durability of the golf shaft.